One known type of die (that is a body, largely composed of semiconductor material, containing electronic circuitry formed therein and which is a fragment of a semiconductor wafer) has an array of electrical contacts on at least one of its major sides. The die is attached to a substrate (typically a circuit board) in the following steps. Firstly, an array of electrically conductive bumps (solder balls) are formed on the respective electrical contacts of the die, to form a flip chip. (Optionally, the solder balls are placed on the wafer before the wafer is singulated into individual dies, thereby forming the flip chips.)
The flip chip is then inverted and positioned with the solder balls on respective bond pads of the substrate, with a small space between the die and the substrate. The solder is then heated to cause re-flow, in which an electrical connection is produced between the balls and the respective bond pads of the substrate. An electrically-insulating adhesive (“underfill”) is then injected into the space and then cured. The underfill provides a strong mechanical connection between the die and substrate, and a heat bridge between them, and ensures the solder joints are not stressed in use due to differential heating of the chip and the rest of the system.
This process is relatively slow because it takes considerable time to fill the space between the die and the substrate. For example, this may take 4 to 5 minutes for a 6 mm die with a 3 mm gap. Since this step is performed individually for each substrate (e.g., each circuit board), it can cause a bottleneck in a process which produces a great many dies from each wafer. Furthermore, the separate curing procedure may take 1-4 hours. Furthermore, air bubbles may be trapped between the die and the substrate, creating “voids”. Also, a lateral force is applied to the solder balls during the insertion of the underfill layer which may displace them. This is expected to become an increasing problem in the future as the pitch of the die pads of the flip chip becomes increasingly fine.
The underfill material is typically a thermosetting material, so once the die and substrate are attached, they are hard to separate again. Choosing the underfill material is subject to conflicting requirements. If the material has high filler loading, it tends to flow slowly into the space between the die and the substrate. Conversely, if it has low filler loading, it becomes subject to “popcorning”. Furthermore, some suitable underfill materials have a relatively short floor life before their curing properties degrade. Also, if subsequent process steps expose the underfill layer to solvents, the solvents may cause voiding or bubbling within the cured underfill.
An alternative attachment procedure is to dispense the liquid underfill onto the bond pads before positioning the flip-chip onto it. That is, the flip-chip is moved towards the substrate, such that the solder balls push the adhesive out of their way, until the solder balls make contact with the respective bond pads of the substrate. Once the solder balls have contacted the respective bond pads of the substrate, a heat treatment process is performed to bond the solder balls to the bond pads and to cure the underfill.
Note that once the bond pads of the substrate are covered by the underfill, aligning the flip chip with them is less than straightforward. Although the alignment may be performed before the underfill material is dispensed, this alignment may degrade when the flip chip is moved towards the substrate, and there is a tendency for the die to “float” on the underfill material (that is for the resistance of the underfill to displace the flip chip from its correct position). Furthermore, underfill material may remain trapped between the solder balls and the bond pads, risking mechanical or electrical joint failure.
Furthermore, this alternative procedure shares many of the disadvantages of the first technique explained above. It is performed at a package level, so it is relatively slow. Also, it too is susceptible to voiding. Also, materials which would allow it to be performed reliably are not yet available. For example, the material should have a coefficient of thermal expansion (CTE) no higher than 80 ppm/K, to ensure that it is not damaged during the heat treatment process, and such materials are not readily available. Also, there is potential for moisture to be absorbed into the underfill, damaging it. Also, once the curing operation is concluded, the resulting package is not reworkable.